Interceptions in the NFL

In a sense, gridiron football is just a fancy version of keep-away — and interceptions are when that game goes awry.

You could have scored points, you could have drained the clock, you could have tired out the defense… But instead, they snatched the ball out of the air and regained control of their own destiny.

When do interceptions occur? And can we use this information to make informed decisions about them going forward? Using NFL play-by-play data from 2009 to 2018, we’ll explore the context of interceptions and see what we find. (You may also wish to explore the Google Colab notebook used to arrive at these conclusions.)

How dangerous are interceptions?

The worst case scenario when you throw an interception is that the player runs the ball all the way back to your endzone, scoring a touchdown: a “pick six.” How often does the happen?

Out of 186,677 passing plays 4,351 were intercepted; league-wide, on any given year, we can expect ~2.3% of our passes to be intercepted. Of these interceptions, 471 were returned for a touchdown.

This means that 10% of your interceptions will lead directly to a score for the other team. In other words, throwing an interception is as good as handing your opponent half a point on a silver platter; not great.

And also note as well that this means about one quarter of one percent of all passes turn into points for the other team. Passing is the key to victory in the modern NFL, but it does not come without its risks.

Literally when do they occur?

There are four quarters to every game of football, and if the score is tied at the end of these 15 minute sessions, a fifth “quarter” called overtime is played. Are interceptions more likely to occur in any particular quarter?

The raw numbers (interceptions / quarter) show that most interceptions happen at the end of the game, in the fourth quarter:

And if we normalize these numbers so that instead we track interceptions / pass / quarter, the trend becomes even clearer: As the game nears its conclusion, the probability of a pass being intercepted goes up (there are all kinds of confounding variables explaining the drop-off in overtime, but these are beyond the scope of this article).

Why this might be the case deserves exploration all on its own; perhaps the defense gets used to offense as the game goes on and learns its patterns; perhaps offenses inherently play riskier as the game goes on; perhaps it takes the nefarious equipment managers a few quarters to plant magnets in all the balls and all the defenders’ gloves. Regardless, one thing is clear: Careful ball handling only becomes more important as the game nears its conclusion.

Where do they occur?

The field of play in gridiron football is 100 yards in length; each team defends one side of the field. Are there some locations on this field that are more conducive to the defense getting an interception? Does more space benefit them, or less?

On pure volume, the 79 yard line appears to be the Big Bad. Interceptions become more frequent as you approach it, and then drop off immediately afterwards:

But adjusting for how many passes were thrown from each yard line reveals a very different picture:

You’re probably going to throw an interception every fifty passes, until you get to the redzone (inside the opponent’s 20 yard line; ie, very close to scoring a touchdown). Once inside, you’re more likely to throw an interception every twenty five attempts, or even every fourteen. Defenses as a whole really step up when backed into a corner — interceptions become three times as likely as anywhere else on the field.

How can we avoid interceptions?

Finally, let’s look at what we can (or cannot) do to prevent an interception. The first thing that comes to mind is the quarterback getting hit as he throws the ball. Surely this would increase the chances of an interception; surely we need to make sure to protect him? As it turns out, this may not be the case:

Quarterbacks are slightly more likely to throw an interception when they aren’t hit. (Perhaps Kyler Murray was onto something when he got sacked by his own lineman…?)

Well, okay, maybe we don’t have to protect the QB as much as we thought. What about starting in a shotgun formation? Normally, the quarterback gets the ball right at the line of scrimmage, right next to the defenders who are trying to sack him. Surely if he starts five or so yards back, increasing the distance between them, he’ll buy himself more time and can make a better throw?

Again, the data (at first glance) does not support this hypothesis:

QBs are ever-so slightly more likely to throw an interception when they start out of shotgun. Are they getting into their own heads? Why are a clean pocket and no dropback detrimental to the turnover game?

Finally, more for fun than anything else (it’s common to make fun of Mitchell Trubisky, for example, for not being able to throw left), let’s look at the direction the pass is thrown. Are interceptions more likely on one side of the field than another?

The middle of the field, historically so difficult for the offense to throw to, apparently remains a bit of a death ground. There is a significantly higher chance of your pass being intercepted between the hashmarks than it is on either sideline.

You know, I’ve learned something today

The NFL is becoming more-and-more pass friendly as the years go on, but we are also fortunate to see a co-evolution in the pass defense as well. The Patriots this year, after less than half the season, have already surpassed their number of interceptions from last year. They have twice as many interceptions as the second-highest team. And in their last primetime game, they had only 11 plays on defense that did not end in interceptions.

If the Patriot’s offense had sat out the entire season, and the team had to stand and fall by their defense alone, they would still be 3–3–1 (they are currently 7–0). The new age of the passing is, for those who understand it and know how to exploit it, a new age of turnovers the likes of which we haven’t witnessed since the Peanut Punch; and given how quickly a single pass can change the game, we may be at the doorstep of the dawning of an entirely new philosophy of gridiron football.

If you can train a defense to consistently intercept passes, all the normal rules go out the window. Do you really need a run game at all if you can depend on a +3 turnover differential every week? Does clock management even matter when your defense can be trusted for seven to fourteen points per game (a swing of 14 to 28 points)? Do you really have to spend 20% of your cap space on a franchise quarterback when your defense alone has a 0.500 win record?

We have here only pulled back a corner of the curtain, and far more research is required to turn interceptions into schemes. But for now, we can use a simple rule of thumb:

For the love of god, do not throw the ball on the one yard line at the end of the fourth quarter with a clean pocket out of shotgun to the middle of the field.